1. Field of the Invention
This invention relates to methods and apparatus for machining a workpiece with a tool and, more particularly, to methods for varying the tool geometry to perform a number of functions including sensing tool wear, enhancing tool life, machining a workpiece of two different materials and suppressing chatter, and apparatus for carrying out these methods and for providing optimal tool geometries.
2. Background Art
In the mechanical processing of materials, the shape, size or properties of a given workpiece are altered. Those processes in which the size and shape are changed by removing material with a tool are known as machining or cutting. In practice, all operations such as turning, boring, drilling, milling, reaming, tapping and cutting fall into this single category of machining or cutting.
In the machining art, a given tool will have a number of surfaces or tool faces, each making a particular angle with respect to the cutting velocity of the workpiece when cutting the workpiece, and these angles specify what is known as the tool geometry. For example, a cutting tool normally has a face known as a rake face, and the angle that this face makes in relation to the cutting velocity is known as the rake angle. A good example of the entire tool geometry for a standard lathe tool is given in Metal Cutting Principles, by M.C. Shaw, the M.I.T. Press, 1968, Chapter 13, to which reference may be had for a better understanding of the prior art and the invention.
The significance of the tool geometry can be shown by the following. In the process of machining the workpiece, different tool geometries are preferred for different cutting conditions so that, for example, one tool geometry is recommended for machining an aluminum workpiece and a different tool geometry is recommended for machining steel. While there are tables describing representative rake and other angles of the tool geometry for machining workpieces of different materials, these angles are not necessarily optimal. For example, for machining a workpiece to a desired surface quality, the optimal angles used are influenced by the individual machining conditions such as the combination of tool material, lubricant, cutting speed, feed rate, and depth of cut, in addition to the workpiece material. Thus, in a finishing operation on a workpiece of a given material, where the depth of cut and feed rate are small, a significantly larger rake angle should be used than in other machining operations on the workpiece such as a roughing operation. The other angles of the tool geometry also would be different in the finishing operation for optimal machining.
In current machining practice, a tool having a particular geometry such as a specific rake face is fixed on a tool holder, whereby the rake angle is fixed and cannot be changed. If, for example, both roughing and finishing operations are to be performed on the workpiece, different tools having different rake faces should be used to provide different tool geometries for these different operations. This, of course, is disadvantageous in that a large number of different tools having different rake and other faces are required in inventory to provide different tool geometries, or additional time and expense are needed in regrinding a given tool face to meet the different tool geometry requirements for different machining operations. More importantly, the idle time associated with changing or regrinding the tool, i.e., the time during which the workpiece is not being cut, can be quite high. This idle time would be even higher if, for example, after a roughing operation, the workpiece were moved to another machine tool for a finishing operation.
The significance of the tool geometry is also exemplified by the following. With other cutting conditions being fixed, the tool geometry will affect the mechanics of chip formation and, therefore, will affect (1) the geometric and other properties of a machined surface and (2) the tool wear behavior and the tool life. The tool geometry will also affect the magnitude and direction of the cutting forces and the damping property of the cutting process; consequently, the tool geometry will influence the static deflection and dynamic stability of a machine tool-machining system.
For example, while cutting the workpiece with a fixed tool geometry, the tool will wear resulting in deviations in the outside or inside diameter of the cut workpiece for lathe cutting or for boring, respectively. One way to compensate this deviation is to move the tool carriage and hence the worn tool into the workpiece at the same rate as the tool wears. However, this can normally be done only by the complex technique of programming this movement into numerical control (NC) machines. Another shortcoming of this approach is that by continuing to cut with a worn tool rather than a sharp tool, the workpiece will experience extra thermal distortion which is difficult to account for. It therefore is desirable to easily determine when the tool has worn and take simple corrective action, if needed.
Also, for example, in boring a hole with a fixed tool geometry in a workpiece having at least two different materials connected at a boundary, problems are encountered which are not only related to the optimum tool geometry that is required for each workpiece material, but also to the vibrations that are induced by impact cutting when the cutting tool moves from one material to the other material of the workpiece. The impact, whose force will depend on the cutting forces which in turn are dependent on the tool geometry, will not only cause tool vibration which will thus reduce the dimensional accuracy of the bored hole, but also will place a limit on the machining speed and material removal rate. For a ceramic cutting tool, the impact may crack the tool and may ruin the workpiece. Still further, with a workpiece of a single material or different materials, what is known as chatter may be encountered while cutting the workpiece with a fixed tool geometry and this too will produce an undesirable surface on the cut workpiece.
In current machining practice, there is no satisfactory technique for performing various functions in the process of cutting a workpiece, such as roughing and finishing a workpiece with a single cutting tool, sensing tool wear, varying cutting forces in magnitude and direction as the tool moves from one material to another of a workpiece to reduce the vibration resulting from the impact, and suppressing chatter to, for example, improve the quality of the workpiece.